In the Rankine steam power cycle it is observed that the condensate which is at low temperature has an irreversible mixing with hot boiler water and this result in decrease of cycle efficiency. To increase the efficiency methods are adopted to heat the feed water entering in boiler by interchanging the heat with in the system. This heating of feed water and in steps is called regenerative feed heat method and the cycle is called regenerative cycle.
Principle of Regenerative power cycle:
The principle is to extract the steam from the turbine at several locations and supplying the steam to the regenerative feed water heaters. This cycle is also called as bleeding cycle. According the capacity of the turbines the heater arrangements for effective efficiency is derived as follows:
- Medium capacity turbines – not more than 3 feed heater
- High pressure high capacity – not more than 5 to 7 feed heaters
- Super-critical turbines – 8 to 9 heaters.
The TS Diagram of regenerative power cycle is shown below:
Regenerative power cycle operation:
The figure shows the layout of steam power plant with regenerative power cycle operation. The surface condenser is used to condensate all the steam extracted from the turbine. The condensed water is allows to flow through the LP and HP heater in stages. The steam extracted from the turbine at different stages is supplied to LP and HP stages to heat the feed water. The conditions of steam bled for each heater are selected so that the temperature of saturated steam will be at 4 to 100 higher than final condensate temperature.
Advantages of regenerative power cycle:
- The heating process in the boiler tends to become reversible.
- The thermal stresses in the boiler are minimized. This is due to the inlet temperature is increased so that the temperature difference is less in the boiler.
- The thermal efficiency is improved because average temperature of heat addition to the cycle is increased.
- The Heating rate is reduced.
- The LP turbine blade height can be reduced because steam flow at lower stage is reduced.
- The steam condenser size can be reduced.
- The turbine efficiency increases and damage to the turbine is less.
Disadvantages of regenerative power cycle:
- The plant becomes more complicated and cost.
- The added heater in the circuit increases the maintenance is required.
- For a given power a large capacity of boiler is required.